Galvanic cells may generate large quantities of gas under certain conditions during use. Since these cells are required to be tightly sealed at all times in order to prevent loss of electrolyte by leakage, high internal gas pressures may develop. Such pressures may cause leakage, bulging or possible explosion of the cell if not properly vented. If a vent valve is employed, it may be resealable in order to avoid drying out of the electrolyte over the life of the cell and to prevent ingress of oxygen from the atmosphere which can cause wasteful corrosion of the anode.
In the past several different types of resealable pressure relief vent valves have been used for releasing high internal gas pressures from inside a sealed galvanic cell. One type of valve that has been commonly used consists basically of a valve member, such as a flat rubber gasket, which is biased into a sealing position over a vent orifice by means of a resilient member, such as a helical spring. The resilient member or spring is designed to yield at a certain predetermined internal gas pressure so as to momentarily relieve the seal and allow the gas to escape through the vent orifice.
Another type of resealable vent is disclosed in U.S. Pat. No. 3,451,690 to Richman issued on Dec. 10, 1968. In this vent, a flat elastomeric seal gasket overlies the vent opening and is retained in place by a resilient terminal cap on the top of the cell. This vent operates in basically the same manner as the vents previously described.
In U.S. Pat. No. 3,664,878 to Amthor issued on May 23, 1972, a resealable vent is disclosed which comprises a resilient deformable ball of elastomeric material positioned to overlie a vent orifice provided within the cell's container. A retainer means is positioned over the resilient ball for maintaining the ball in place over the vent orifice and in contact with a valve seat provided around the peripheral edge portions of the vent orifice and for compressing and deforming the resilient ball into a flattened configuration forming a normally fluid-tight seal between the flattened ball and the valve seat. The resilient ball is capable of undergoing further temporary deformation upon the buildup of a predetermined high internal gas pressure inside the container so as to momentarily break the seal and allow gas to escape through the vent orifice.
Alternates to high pressure resealable vent means are fail safe venting systems as illustrated in U.S. Pat. Nos. 3,218,197 and 3,314,824. Specifically in the '197 patent a venting means is disclosed in which the seal gasket has a thin section that will rupture or "blow-out" at a predetermined high pressure buildup within the cell. The difficulty with this type of venting means is that for small diameter cells it is difficult to obtain consistency in the thickness of the "blow-out" section of the gasket using conventional manufacturing techniques. In the '824 patent a puncture-type safety seal is disclosed which comprises a spring washer positioned within the cell's seal assembly and a radially acting toothed puncture washer. The teeth of the washer slide relative to the spring washer when the spring washer is subjected to internal pressure so that at a predetermined gas pressure buildup, the teeth of the washer will puncture the seal's gasket thereby providing a vent passage. This type of venting means requires several component parts, is rather expensive to make and assemble, and is not suitable for small diameter cells.
U.S. Pat. No. 4,079,172 discloses sealed galvanic dry cells having at least one gas venting passage disposed at the interface of the top surface of the cover and then curled over the rim of the container. The passage is defined as being a recess in the cover disposed below the curled over rim and/or a notch in a portion of the curled over rim.
U.S. Pat. No. 5,227,261 relates to electrochemical cells which are comprised of a seal member that has a centrally located cylindrical hub joining a base, which base has a ventable diaphragm portion and a nonventable diaphragm portion, which hub has an end extending above the base and an end extending below the base, into which a current collector is inserted in an interference fit with the end extending above the base, which ventable diaphragm portion joins the hub at an interface forming an arc of between about 135 degrees and 250 degrees, and which interface is the thinnest portion of the base.
U.S. Pat. No. 4,255,499 relates to a galvanic cell in which a first arc portion between about 150.degree. and 190.degree. of the top peripheral edge of the container is turned or curled over the container's closure means so that when a predetermined high pressure build-up develops in the cell, the pressure will cause the closure means to tip or tilt about the diametral axis formed between the first arc portion and the remaining second arc portion so as to produce a vent passage at the interface of the container and the closure means proximal the remaining second arc portion of the top peripheral edge of the container.
As discussed above, resealable, high pressure relief vent valves are generally bulky and/or difficult to incorporate into a cell assembly, bulky type of blow-out safety vents are not suitable for small cell applications, while low pressure vent means for some cell systems may not adequately and sufficiently prevent loss of electrolyte through leakage or prevent ingress of oxygen from the atmosphere which could cause wasteful corrosion of the anode.
It is, therefore, an important-object of this invention to provide a compact and economical pressure vent for use in a galvanic cell that will effectively occupy minimum volume so that maximum volume of the cell can be used for the active components of the cell.
Still another object of this invention is to provide a predetermined pressure vent using a snap-through gasket for galvanic cells which is easy to produce and inexpensive to manufacture.
Another object of the present invention is to provide a vent which prevents premature release of gas pressure from within a galvanic cell.
The foregoing and additional objects will become more fully apparent from the following description and the accompanying drawings.